1. Field of the Invention
The present invention relates in general to a hydraulically operated brake system for an automotive vehicle, and more particularly to a manually/electrically operated brake system wherein a braking effect can be electrically controlled, and which is capable of applying an effective brake to the vehicle even in the event of an electrical failure of the system.
2. Discussion of the Prior Art
A hydraulically operated brake system is generally used in a motor vehicle as a service brake for decelerating or stopping the vehicle. This hydraulic brake system includes a master cylinder, a wheel brake cylinder, and a fluid passage connecting the master cylinder and the wheel brake cylinder. The master cylinder is adapted to generate a manually controlled braking pressure according to an operating of a brake operating member such as a brake pedal. The wheel brake cylinder receives the manually controlled braking pressure, and applies a brake to a wheel of the vehicle, for restraining the rotation of the wheel. In this type of manually operated brake system, the required operating stroke and force of the brake operating member are determined by an amount and a pressure of the brake fluid that is necessary to provide a desired braking force (hydraulic force provided by the wheel brake cylinder). The required operating stroke or force of the brake operating member may be reduced if a booster is provided between the operating member and the master cylinder. However, the reduction in the operating stroke is limited, and the braking force applied to the wheel is always commensurate with the operating force applied to the brake operating member. In the manually operated brake system, therefore, the operating feel of the brake operating member cannot be adjusted as desired.
Laid-open publication No. 58-188746 of Japanese Patent Application (published in 1983) discloses a hydraulically operated brake system for a motor vehicle, wherein a braking force applied to the vehicle wheel is not necessarily proportional to an operating force applied to a brake operating member. It is recognized that a rate of deceleration of the vehicle upon brake application to the wheels is not determined solely by the braking force generated by pistons of the wheel brake cylinders. Namely, the deceleration rate is affected by other factors such as a friction coefficient of a friction member of the wheel brakes, and a load on the vehicle. To obtain a consistent deceleration rate of the vehicle varying with the operating force, without being influenced by the other factors as indicated above, it is necessary to avoid direct proportioning of the braking force to the operating force applied to the brake operating member. To this end, the brake system disclosed in the above-idenfified document uses sensors for sensing the operating force applied to the brake operating member, and an actual rate of deceleration of the vehicle, and includes an electrically controlled actuator which is adapted to control the braking pressure to be applied to the wheel brakes, depending upon the detected brake operating force and deceleration rate of the vehicle. Described more specifically, the boosting ratio of a booster provided between the brake operating member and the master cylinder is electrically controlled to adjust the pressure of the brake fluid pressurized by the master cylinder and consequently the braking force generated by each wheel brake cylinder. Alternatively, an amplifier is disposed in a primary fluid passage connecting the master cylinder and the wheel brake cylinders, and the boosting ratio of the booster is electrically controlled to adjust the braking force generated by the wheel brake cylinders.
In the above hydraulically operated brake system, however, the braking pressue applied to the wheel brake cylinders is necessarily derived from the pressure generated in the master cylinder, which is merely electrically adjusted by means of the amplifier. Hence, the braking force generated by the wheel brake cylinders has a close relation with the operating stroke and force of the brake operating member. This indicates that the freedom to adjust the operating feel of the brake operating member is inevitably limited.